Electron microscopes in general and scanning electron microscopes (SEM) in particular use an electron beam probe to examine specimens. The electron beam requires a good vacuum, where it is generated by an electron gun source and propagated through focussing lenses all the way to the specimen. In addition, many of the detection means used to detect the emerging signals from the beam-specimen interaction also require a vacuum condition, in which, however, the specimen is severely limited. In the past, this meant that only dehydrated specimens could be used. In addition, because the electron beam delivers an electric current, the specimen should generally have a conductive surface to prevent accumulation of charge that hinders normal operation of the instrument. This meant that generally insulating surfaces could not be examined. However, the more recent technology of environmental scanning electron microscopy (ESEM) has made it possible to examine specimens in a gaseous environment. The presence of a gaseous envelope around the specimen at sufficient pressure makes it possible to maintain moist conditions so that hydrated specimens can be observed in their natural state. Also, the ionised gas dissipates the electron beam current away from the surface of insulating specimens and, therefore, these specimens need not have the pre-treatments conventionally used to render their surface conductive. Furthermore, the gas is used as detection medium to detect the signals propagated and amplified in the gaseous envelope around the specimen. Such signals are usually the secondary electrons (SE) and the backscattered electrons (BSE) from the specimen, which ionise the surrounding gas and amplify the signal that is processed by appropriate means to form and display images or sprectra. However, certain disadvantages on the field of view need still to be overcome.
A particular prior art by U.S. Pat. No. 6,809,322B2 patent provides, among other disclosures and advantages, the possibility of using a relatively small pressure limiting aperture (PLA) without restricting the field of view. The basis of claims for the said art is the employment of beam deflection elements, all of which are confined between two PLAs; one aperture (PLA1) is located at the end of the electron optics column, while the beam is rocked by said deflection elements with a pivot point at or near the plane of PLA1. However, the claim of the necessity to locate and confine all deflection elements between the said two PLAs imposes certain limits on the scope, performance and industrial applicability.
Another particular prior art by U.S. Pat. No. 5,362,964, among other claims, provides also two PLAs but with two beam deflection elements located before the beam enters the space between the two PLAs plus a third deflection element located between the two PLAs. The first aperture (PLA1) is located at the end of the electron optics column, whilst the second aperture (PLA2) together with the third deflection element are located inside a magnetic lens with reduced focal length. The said third deflection element in combination with the magnetic lens field deflect and rock the beam with a pivot point at or near the plane of PLA1, so that the beam scans a large field of view at the specimen plane. However, the latter combination or configuration has posed a serious doubt on its practical feasibility and industrial applicability; notably, the full life of that patent has expired without any laboratory tests published or any commercial implementation.
Yet another prior art by U.S. Pat. No. 8,405,045B2 patent also provides multiple sets of deflection elements to achieve a large field of view at the specimen level: The beam is rocked with a pivot point at a final aperture, which, however, is clearly located inside the objective lens at or near its principal plane for the purpose of preventing electron beam aberrations and preserve resolution and usability. However, the latter location of the pivot point imposes a serious limitation on the scope and performance in the presence of a gaseous environment in an ESEM type of instrument, which requires the placement of a PLA1 at the end of the electron optics column for optimum performance. The latter requirement has not yet been achieved industrially since the inception and commercial exploitation of ESEM technology.